(a) Field of the Invention
The present invention relates to an improvement of an optical system for ophthalmological instruments such as fundus cameras.
(b) Field of the Invention
The conventional fundus camera has such a composition as shown in FIG. 1 wherein the reference numeral 1 represents an eyeball to be examined, the reference numeral 2 designates an objective lens, the reference numeral 3 denotes a mirror having an aperture, the reference numeral 4 represents a stop, the reference numeral 5 designates a relay lens, the reference numeral 6 denotes a switching mirror, the reference numeral 7 represents an eyepiece, the reference numeral 8 designates a film surface, the reference numeral 9 denotes a light source for observation, the reference numeral 10 represents a heat-preventive filter, the reference numeral 11 designates a photographing light source such as strobo tube, the reference numeral 12 denotes a ring slit, reference numerals 13 and 14 represent collector lenses, and the reference numeral 15 designates a lens for projecting an image of the ring slit. In this optical system, the light source 9 is always lit and emits rays which are focused by the collector lens 13 so as to form an image of the light source 9 at a position substantially the same as that of the photographing light source 11. This image of the light source is focused again by the collector lens 14 at a position in the vicinity of the ring slit 12. Therefore, the ring slit functions as a secondary light source in the optical system arranged at the subsequent stage. The rays having passed through this ring slit are projected by the projector lens 15 to a position in the vicinity of the mirror 3 having an aperture. This image of the ring slit is further projected by the objective lens 2 to the cornea surface of the eyeball to be examined to achieve ring-shaped illumination already known to those skilled in the art. The illuminating rays scattered by the fundus illuminated in this way are projected through the cornea and focused by the objective lens 2 so as to form an image of the fundus at a position between the objective lens 2 and mirror 3 having an aperture. This image of the fundus is passed through the stop 4, focused by the relay lens 5 and reflected by the switching reflector mirror 7 and reflector mirror 7a, and formed again on a focusing plate 7b for observation through the eyepiece 7c. When the switching mirror 6 is turned to the position indicated by the chain line, the image of the fundus is formed on the film surface 8 for photographing.
In order to design a fundus camera comprising such an optical system more compact, there can be considered a method to design the lenses more compact, another method to shorten the total length of the lens system as a whole, a third method to shorten focal length of the lens system as a whole and so on. However, all of these methods offers possibility to design the optical system compactly only within a certain limited range.